Saturday, 24 January 2009

My new extruder has a 0.3mm nozzle compared to 0.5mm that I have used before. The actual filament diameter is controlled by the flow rate versus the head feed rate, so a single nozzle can give a range of filament diameters.

The maximum diameter is governed by the hole size and the die swell. The head movement has to be about the same as the rate that the filament leaves the nozzle, or faster, otherwise the filament squirms about and makes a zigzag instead of a straight line. Fortunately the faster the flow rate, the more die swell there is, which works in our favour when trying to extrude the maximum diameter filament. With the 0.5mm nozzle I could extrude up to about 1mm with ABS and I used that to good effect when making the first layer of the raft. With a 0.3mm hole die swell is more but even so I can only get 0.8mm filament. That makes the first raft layer thinner, so it is less tolerant to the bed being uneven.

I normally extrude at a rate that produces filament the same diameter as the nozzle but it can be stretched further making it smaller than the nozzle. The limiting factor is when the filament starts to snap. I did make some 0.3mm filament with the 0.5mm nozzle but I don't think I got the full benefit of the extra resolution because the filament was less constrained as the nozzle changed direction.

These two gears are both made from the same gcode with 0.3mm filament giving a layer height of 0.24mm. The one on the left was made with a 0.5mm nozzle and the one on the right with the new extruder with 0.3mm nozzle. The latter is slightly better defined. The benefit is more apparent on the underside.

The bottom of the one on the left feels perfectly smooth due to being made on a raft with a very fine surface. It is actually smoother than a sample I have from a commercial machine.

I was disappointed that it did not improve the clockwise slant of the teeth. This must be due to the same effect that makes holes come out too small. The filament likes to cut corners, so when the head moves on a curved path the filament takes a smaller radius path. I noticed that the teeth are straight at the base but slanted at the top, so the effect is somewhat cumulative.

I made another one with the outlines anti-clockwise on every second layer. Here is a video of it being made: -

The teeth came out straighter but the edges are slightly more ridged because each layer alternates a little.

The surface is not quite as good as the previous one. I put that down to variations in the feed stock diameter. You need exactly the right amount of plastic to get a good surface.

I also need to up the resolution of my z-axis. 0.05mm is significant with 0.24 mm layers, so I will have to add microstepping like my other axes.

So in summary 0.3mm nozzle gives noticeably better results and can still make 0.5mm filament due to die swell. It is harder to get the raft heights and temperatures correct. To get the same build rate with 0.3mm filament I would have to extrude at 44mm/s, but HydraRaptor is currently limited to 32mm/s. I could probably tune it up to 44 but the vibration gets a bit ridiculous as the moving mass of the table is 9Kg.

Saturday, 17 January 2009

The ideal off the shelf heater would be a cartridge heater but they tend to be at least 1" long, need mains voltage and are very expensive. Here is a cheap 12V alternative: -

It is a vitreous enamel wire wound resistor that can handle surface temperatures up to 450°C. It is a 6.8Ω RWM 6 x 22 rated at 10W, but I am overloading it somewhat to get 240°C.

I bought a pack of five from RS. Farnell and Newark also stock them I believe.

I drilled a hole to accept it in a 19 x 19 x 8mm block of aluminium with an M6 tapped hole for the heater barrel and a small hole for a thermistor.

The tapped hole is at right angles so that the hot zone is as short as possible. It could be made parallel to get more contact area.

The outside diameter of the resistor measured 6.3mm so I drilled a 1/4" hole for it. That was too tight so I drilled it out to 6.5mm. I then wrapped aluminium kitchen foil around the resistor to make it a tight fit and rammed it in.

Here it is under test with a random bit of tube to simulate a heater barrel.

It needs about 11W (8.7V) to get to 240°C. 14.7W (10V) gives 300 °C. I haven't run it for very long so no guarantees it will last, but I can't see why not.

Compared to the aluminium clad resistors I tried before, these are cheaper and you get a more compact heater with a smaller surface area to lose heat from. Also making connections should be no problem with normal solder because the wires are long enough to cool down.

I have not succeeded yet in getting the stainless steel barrel to extrude easily, so I had a go at improving my aluminium extruder to remove the need for a fan.

A lot of heat is lost from the large flange on the top of the heater barrel and transfers by convection to the heatsink above and by conduction through the bolts.

When I stripped it down I noticed the bolts had loosened.

The PTFE had shrunk lengthways and expanded in diameter and was no longer making a good compression seal.

Although no plastic had escaped it had leaked under the PTFE.

I think it was leaking so slowly that it oxidized where it met the air and went hard, stopping further flow. I hadn't run it very long so it may have escaped eventually.

So PTFE is obviously no good in compression at these temperatures. I replaced it with PEEK, which is a shame because it is about ten times more expensive.

I also replaced the aluminium flange with an M8 x 25mm steel washer insulated from the barrel by a PEEK collar.Here are the parts, the PEEK section is drilled in situ to get perfect alignment: -

And here it is assembled: -

I put some PTFE plumbing tape over the hot end of the PEEK before pushing it into the aluminium in an attempt to improve the seal.

The heatsink now runs at 80°C without the fan. I would have liked it to be lower but as long as it is below the glass transition of the filament and the clamp it should be OK. I tried insulating the bolts with PEEK washers but that only dropped the temperature by 5°C, so not really worth the effort.

After one heat cycle I noticed the bolts were not as tight as they should be so it looks like PEEK creeps a bit as well. Perhaps glass filled would be better.

It is disappointingly complex with lots of machined parts but it does work very well. The heater power has dropped to 50% from about 80-90% with the fan. ABS filament extrudes manually very easily and even HDPE only requires moderate force. I think actually having the long heatsink preheating the filament to just below the glass transition is probably a benefit.

It is too early to say whether this design will be reliable but other than the PEEK section leaking there isn't anything likely to fail. I don't mind making things that are difficult to make provided I only have to do it once.

I also have a much simpler design in mind that should achieve the same short transition zone.

Sunday, 11 January 2009

Although my last extruder design seems to work, I am not very happy with it. I don't like the little fan because it is noisy, it isn't easy to make and it is not very thermally efficient. The main heat loss is via the stainless steel bolts and from the big flange. The only reason those parts are needed is because the PTFE insulator does not have the necessary mechanical strength and stability.

Some time ago I tried to use stainless steel as the insulator because it is strong, self supporting and withstands high temperatures. That attempt failed because my thermal gradient was too long; the hot to cold transition was about 50mm. The extruder would run for a while but would always jam before an object was completed. Once the stainless steel barrel was fully up to temperature the amount of filament that is soft but not fluid is sufficient to provide too much resistance to be pushed. This theory was confirmed when I tried a soldering iron heater, which also has a long thermal gradient along its length.

I also tried PEEK as the metalab guys had success with it but that seemed to suffer from the same problem.

I had always intended to revisit the stainless steel idea with a shorter transition zone but when I saw Larry Pfeffer's stainless steel extruder it provided an extra idea of thinning down the pipe at the transition. That allows a short transition without too much heat loss or loss of mechanical strength.

I did some experimentation with this test set-up.

I made a heater with an integral aluminium barrel by turning some aluminium bar in my four jaw chuck, the first time I have used it. I used two 12Ω resistors in parallel this time instead of one 6.8Ω. They give a bit more power and possibly lower internal temperature inside the resistors.

I measured the temperature along the tube at 5mm intervals. The thermocouple is slightly smaller than the internal diameter of the tube. The weight of its cable causes the tip to rest on the roof of the tube and the other end rests on the floor of the filament exit. The thermocouple is itself encased in stainless steel, so there will be some heat leaked along it. Hopefully its casing is thin enough to have little effect compared to the much thicker tube it is sampling.

The heater was powered from a bench power supply and the voltage adjusted manually to give around 240°C in the middle of the heater block. That needed 9.4V which is 14W. I can feel substantial heat rising from it so some insulation would make it a lot more efficient. I have got some ceramic fibre kiln insulation for that, another tip I picked up from Larry's blog.

The first test was with a threaded tube with no constriction. The gradient is not far from linear, it falls off faster when hotter due to more convection and radiation from the hot section. If we assume ABS would be soft but very viscous between say 75°C and 125°C we see that it covers 45 to 60 i.e. 15mm.

I then turned a 10mm section of the tube down from 6.4mm to 4.5mm. The internal diameter of the tube is about 3.6mm so that gives a wall thickness of 0.45mm. That made the gradient steeper between 35mm and 40mm but the length of the perceived problem zone gets bigger. I am not sure how Larry gets away without a heatsink. I think he is using thicker pipe so there is a much bigger difference between the conduction of the constricted section and the rest. Also it takes a long time for the problem to become apparent because heat travels slowly down the pipe.

The final test was done with a heatsink attached just above the constriction. The centre of the heatsink only reached about 28°C. The aluminium block I used to connect it got hotter but was still comfortable to touch so less than 50°C.

The gradient between 30mm and 40mm is now much steeper. Odd that it is not between 25mm and 35mm where the constriction is. Almost like there is a 5mm offset in the readings. Anyway the 125°C to 75°C transition is now only about 3mm.

Friday, 9 January 2009

The heater in my last design has two layers of Cerastil that take 24 hours each to cure and the bobbin takes some time to machine. Attaching the wires and winding the coil is quite fiddly. Looking for a short cut I wondered if we could use power resistors. I had this one lying around to play with.

Unfortunately it is only rated for operation up to 200°C. In fact the datasheet says "It is essential that the maximum hot spot temperature of 220°C is not exceeded". Curious to see why, I put enough voltage across it to heat it up to 240°C. That turned out to be about 75W. It seemed quite happy at that temperature for several hours.

It is too big really for an extruder so I bought some smaller 10 Watt ones for £1.42 each.

These are only rated for 165°C but what the heck. I heated a 6.8Ω one to 300°C. At about 180°C it produces a little smoke but that soon goes. At 280°C it starts to smell bad, but at 240°C it seems happy. I left one powered up for a few days. The writing disappears and the connection tags oxidise, but its resistance is stable.

To make a heater I cut a 19mm x 19mm x 8mm block of aluminium from a bar, drilled a 5mm hole through it and tapped it to M6 to fit a heater barrel. The mounting holes of the resistor are big enough for M2.5 but there is not enough room for the head, or a nut. M2 is a bit weedy so instead I used 8BA bolts. These need a 2.8mm hole for tapping. A simpler solution would be to just file a flat on the head of an M2.5 bolt, drill a clearance hole and use a nut on the other side.

Here is the full assembly: -

I put heatsink compound on the M6 thread and under the resistor. I attached tinned copper wires with 300°C solder and insulated them with PTFE sleeving.

I have run the assembly for a couple of days and it held up. I am loath to recommend something which is unsound engineering, but it does seem a simple and robust solution as long it lasts a reasonable amount of time, say 1000 hours. Replacement is easy because the most time consuming thing is making the block which is reusable. I expect there might be some matching crimp connections to avoid the high temperature solder.

Quite a lot of heat is lost from the large surface area so some insulation would be a good idea.

Sunday, 4 January 2009

The RepRap design has always aimed to be cheap and easy to make from readily available materials. What I desire though is good performance and reliability, and put those priorities ahead of the others. To me they are absolute requirements and the others are things to be optimised afterwards. With that in mind I set about trying to design a reliable extruder that I can make with the tools and materials I have available.

As it is experimental I wanted it to be modular so I can swap out things that don't work. I started with the heater. It takes me two days to make one so I wanted it to be removable and reusable. I made an aluminium bobbin with an M6 thread through the middle of it so it can be fitted to different barrel designs. The outside diameter is 12mm and the inner diameter is 8mm. It is also 12mm long. The flanges are 2mm and 3mm with a 7mm gap for the nichrome and Cerastil.

The surface is roughed up to make the Cerastil adhere well. It has a hole to accept the thermistor to make it a self contained closed loop.

I put down a layer of Cerastil about 0.5mm thick using a plastic jig and left it to cure over night.

I used two strands of 0.1mm nichrome in parallel to make the heater. That only needs 90mm to make about 6Ω. I normally use 8Ω but I anticipated more heat loss in this design.

To make connections to the heater I used two strands of 0.2mm tinned copper wire and attached them with reef knots.

I then covered the knots in high melting point solder.Using such fine copper wire may be a mistake as Bert pointed out on my previous post. Time will tell.

I made a jig to keep the wire taught while winding it on the bobbin.

At this diameter it is only about three turns of nichrome.

Finally I covered the windings in Cerastil H-115 and also used it to glue in the thermistor.

I made the barrel as short as possible. That turned out to be 25mm to have room for the heater and the nozzle and a mounting flange. The standard design uses a 45mm heater barrel.

The vaned section is a heatsink to keep the rest of the filament path cool. Sandwiched between the hot and cold sections is a 12mm length of 10mm diameter PTFE tube.

The idea is to keep the thermal transition as short and slippery as possible to make it easy to push the slightly molten plastic through. The PTFE extends 5mm into the heatsink to give a good contact area for cooling. It extends 2mm into the hot barrel and 5mm is in the air gap. It is an interference fit and is under compression. When it gets hot and expands the seal should only get tighter.

The metal parts were drilled to 3.3mm on the lathe and once assembled it was all drilled out to 3.5mm. As the PTFE was drilled in situ the hole is perfectly aligned and there are no gaps.

The thermal loss through PTFE, which has a conductivity of 0.25 W/m°C, will be: -

220 × 0.25 × π (0.0052 - 0.001752) / 0.005 =0.76W, assuming the heatsink is at 20°C and the barrel is at 240°C.

The barrel is held on by three M3×25 stainless steel bolts. The holes are counter bored so only the last 5mm of thread is in contact with the heatsink. Assuming the mean diameter of the thread is 2.75mm the heat loss through the bolts is: -

3 × 220 × 17 × π × 0.0013752 / 0.02 = 3.3W

Longer bolts could reduce this by about half.

Here it is with the heater, nozzle and PTFE cover installed. There is heatsink compound between the heater and the barrel, and the nozzle thread is sealed with PTFE tape.

The wires are insulated with PTFE sleeving and terminated to a 0.1" header mounted on a scrap of Vero board. This mates with an old floppy drive power connector. I put the thermistor in the middle and the heater on the outer contacts so it doesn't matter which way round the connector goes.

The clamp seems to grip aluminium a lot better than it does PTFE but I also put an M3 bolt into a blind tapped hole to ensure it cannot slip. A good move as it turned out.

I powered it up without the pump and calibrated the thermistor. With the nozzle at 240°C the "cold" section reached 100°C and softened the ABS clamp. Obviously my home made heatsink is woefully inadequate.

To keep it cool I added a small fan. That keeps the cold section at 30°C, much better.

The black sheet is Teflon baking parchment that I used to stop the fan blowing on the hot section.

I haven't attached the motor yet but I have tested hand feeding white, green and black ABS as well as HDPE. The ABS feeds easily through the 0.3mm nozzle and the HDPE with moderate force. I think they will all work well with the motor drive.

When the filament is pulled back out only a few millimetres has expanded at the end. In contrast, without the fan the filament swelled most of the way to the top and jammed. You can see the difference here: -

Keeping the melted section short is the key to making the filament easy to feed. The other improvement is that the PTFE is no longer a structural element. It is held in compression and appears to make a good seal with simply a push fit.

I am sure I can both improve the thermal separation and make it easier to make with a couple of design iterations before redesigning the other half of the extruder.